**5. Gypsum**

Agricultural gypsum, 10CaSO4 .2H<sup>2</sup> O, a by-product of the fertilizer industry, originates from the reaction between sulfuric acid and phosphate rocks used to produce phosphoric acid. Gypsum applied to soil does not neutralize soil acidity but decreases aluminum saturation and increases base saturation of the subsurface, providing conditions for a further development and deepening of the sugarcane root system. It is recommended to apply gypsum when CaC2+ contents are lower than 0.4 cmolc dm−3 and/or aluminum saturation is greater than 20% at the 20–40 cm layer. The application of gypsum will lead to the improvement of the root environment at layers below arable ones, an effect that lasts for several years. For this reason, the annual reapplication of gypsum is not necessary. In areas with sugarcane straw or organic residues on the soil, and if the contents of Ca2+ are not very low and/or aluminum saturation is not very high, the response to gypsum may be lower.

analysis, a relation between exchangeable calcium and sugarcane root system was found:

have reported that under conditions of low availability of calcium in the soil, sugarcane roots concentrated at the layer 0–30 cm. However, in this study, 50% of the root system mass was

Soil calcium and magnesium contents decrease during sugarcane cycles both by the removal of bases by harvests and by acidification caused by nitrogenous fertilizers. This effect is demonstrated in the long-term study (**Figure 4**) conducted by [15, 16]. These authors evaluated the

Initially, the soil presented, at the layers 0–20 and 20–50 cm, a base saturation of 15 and 7%, respectively. At the time of preparation of the soil for planting sugarcane, 2.5 t of limestone and 1.5 t of gypsum were applied per hectare. Soil chemical changes in plant cane and regrowth are shown in **Figure 4**. After plant cane thinning, base saturation at the layers 0–20 and 20–50 cm was, respectively, 52 and 38%; by the fifth cut, the values were similar to those

The authors of this chapter have recommended liming for regrowth areas when there is a base saturation of less than 50% at the 0–20 cm layer. The application of corrective should be in the total area preceding crop treatments and calculating the necessary amount as previously

**Figure 4.** Changes in the base saturation of a soil cultivated with sugarcane. Source: adapted from [15, 16].

and the root mass was 1.1 g/dm3

. Several authors

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Mineral Nutrition and Fertilization of Sugarcane http://dx.doi.org/10.5772/intechopen.72300

/dm3

at 150 cm depth, Ca2+ was 0.60 cmolc

observed at the time of reforestation.

described.

**6. Liming in sugarcane regrowth areas**

reacidification of a soil cultivated with sugarcane by five cuts.

in the layer 51–150 cm (**Table 2**).

The doses of gypsum to be applied may be based on the need for liming, or on soil texture. The amount of gypsum to be applied varied between 25 and 30% for the need for liming, multiplied by a depth correction factor (profile to be corrected/20). For example, the amount of limestone to be applied was 3.0 t ha−1, and improvement of the root environment at the 20–60 cm layer is desired. Then, the amount of gypsum will be equal to 1.5 t ha−1[(3.0 x 0.25) x (60–20)/20]. When the doses of gypsum to be applied are based on soil texture, the following recommendation can be used [8]: dose to be applied (kg ha−1) = clay (g kg−1) x 6.0.

Gypsum is applied in total area and may or may not be incorporated into the soil. When it is not possible to use it, mainly because of difficulty in acquiring it in small quantities, a fact that usually happens with micro and small farmers, one should choose to apply simple superphosphate as a source of phosphorus because this fertilizer contains calcium sulfate. In a study conducted by [15], limestone and gypsum rates were studied in a sugarcane crop cultivated in medium texture soils with a low cation exchange capacity. A relation between calcium levels in the soil and growth of the root system was also observed. Twenty-seven months after the beginning of the study, in a treatment with the application of 2.8 t of gypsum per hectare, the highest yield of biomass and industrializable shoots occurred. By soil


**Table 2.** Calcium content in the soil and growth of sugarcane root system in a soil that received limestone and gypsum.

analysis, a relation between exchangeable calcium and sugarcane root system was found: at 150 cm depth, Ca2+ was 0.60 cmolc /dm3 and the root mass was 1.1 g/dm3 . Several authors have reported that under conditions of low availability of calcium in the soil, sugarcane roots concentrated at the layer 0–30 cm. However, in this study, 50% of the root system mass was in the layer 51–150 cm (**Table 2**).
